It's a small price to pay, but Boston sent out three second-round picks (two to Portland, one to Houston) as part of the deal to land Lee. According to the Boston Herald, the Celtics will send Charlotte's second-round pick (acquired last month from Oklahoma City as part of Jeff Green compensation) to the Rockets, while the Blazers get the second-round picks belonging to Boston and Minnesota (acquired from Cleveland in the deal that sent Luke Harangody and Semih Erden to the Cavaliers at the 2011 trade deadline).
An updated glance at Boston's future draft picks (with a tip of the cap to Real GM for past help compiling this list):
2013 - 1 Round 1 pick (own); 0 Round 2 picks
2014 - 1 Round 1 pick (own); 2 Round 2 picks (own, Brooklyn via MarShon Brooks trade)
2015 - 1 Round 1 pick (own); 2 Round 2 picks (own, Sacramento* via Sam Cassell trade)
2016 - 1 Round 1 pick (own); 1 Round 2 picks (own)
2017 - 1 Round 1 pick (own); 2 Round 2 picks (own, Sacramento* via Marquis Daniels trade)
* Top-55 protected; received only if pick is outside top 55.